In power control and motor control for industrial equipment, electric railway vehicles, automobiles, and home appliances, and the like, power transistor modules obtained by mounting a single module with a plurality of switching elements, such as power MOSFETs (Metal-Oxide-Semiconductor-Field-Effect-Transistors) and IGBTs (Insulated-Gate-Bipolar-Transistors), and semiconductor elements, such as freewheel diodes, are used.
Such a power transistor module is configured by: placing a conductive pattern in a predetermined shape over one surface of a heat sink of metal or the like with an insulated substrate in between; mounting a semiconductor element over the conductive pattern by soldering or the like; and electrically connecting the semiconductor element to a terminal for external connection (external lead-out terminal). Hereafter, an insulated substrate and conductive patterns will be collectively referred to as a circuit board.
With respect to such a power transistor module as mentioned above, a plurality of semiconductor elements connected in parallel over a conductive pattern in a predetermined shape within the module may be used so that a large current can be handled. However, when electrical parallel connection is achieved by simply arranging a plurality of semiconductor elements in parallel, a problem arises. The length of a current path to an external lead-out terminal differs depending on the placement position of a semiconductor element over the conductive pattern. As a result, a current value is different from semiconductor element to semiconductor element connected in parallel and a current is converged on some of the semiconductor elements.
When there is variation in current amount among semiconductor elements, the quantity of heat generated from each semiconductor element varies. As a result, a solder joint connecting a semiconductor element to a conductive pattern is different in deterioration from element to element. When a semiconductor element generates especially much heat, a solder joint thereto is most noticeably deteriorated and develops poor connection first in the circuit board, a current being reduced. For this reason, current convergence on another semiconductor element occurs and in the semiconductor element largest in current amount, poor connection is developed in the solder joint thereto for the above-mentioned reason. This eventually leads to a defect event in which the number of elements through which a current can be passed over the circuit board is gradually reduced. As mentioned above, this problem is caused by a difference in the length of a current path between a semiconductor element and an external lead-out terminal, in other words, a difference in impedance.
An example of a technology for reducing variation in impedance in the path from a semiconductor element to an external lead-out terminal is Patent Literature 1. Patent Literature 1 discloses a semiconductor device. In this semiconductor device, an internal junction is provided with a slit to equalize impedance in the path from each semiconductor element to an external lead-out terminal. At the same time, the internal junction is also used as a heat radiation structure. As a result, thermal stress on a bonding wire connecting a semiconductor element and the internal junction can be mitigated.
The semiconductor device in Patent Literature 1 includes: a case; a plurality of semiconductor circuit boards placed in the case and configured by mounting a semiconductor element over an insulated substrate provided with a conductive pattern; an external lead-out terminal for taking a principal current out of the case, a part of the external lead-out terminal being extended into the case; and an internal junction placed along the plurality of the semiconductor substrates and electrically coupled with a main electrode of the semiconductor element, the internal junction including a heat radiation means for maintaining a heat radiation property and a slit structure equalizing the length of the current path from the semiconductor element.
Patent Literature 2 discloses a technology for providing an emitter pattern portion over a circuit board with a slit for sectioning the width of a current path to form an inductance forming region.
According to the power transistor module in Patent Literature 2, even in a wide variety of transistor modules different in specifications, provision of a proper internal wiring inductance can be easily coped with by changing a wire connection point on a circuit pattern without changing a terminal component.